Research by the NHTSA (National Highway Traffic Safety Administration) in the United States has revealed that children in a rear-facing child seat on the front passenger's side of a vehicle have suffered fatal injuries due to air bag deployment. The same thing has occurred with children sitting in the passenger seat too close to the air bag deployment unit. These fatalities could have been prevented by deactivation of the passenger-side air bag. There are also situations in which activation of the passenger-side air bag would be superfluous, e.g., when the passenger's seat is occupied by an object instead of a person.
To be able to identify different types of occupancy of the passenger's seat when the passenger-side air bag must not be activated, various systems are available for sensing seat occupancy. One very expensive option would be an image processing system that could detect whether the passenger's seat was occupied by a child's child seat, a person or an object and could also sense the distance between the person and the air bag, so the air bag could be deactivated in critical types of occupancy. One such method of identifying seat occupancy based on the image processing principle is known from German Patent No. DE 44 00 664 C2, for example.
Another principle for identification of seat occupancy is described in the Proceedings of the Third International Symposium on Sophisticated Car Occupant Safety Systems, Air Bag 2000, Nov. 26/27, 1996, Karlsruhe (Germany), pages 18-1 through 18-13. The seat occupancy sensor presented in this document is a mat integrated into the passenger's seat so that its electric resistance changes as a function of a force or pressure acting on it. The mat thus has the function of a weight sensor to ascertain whether an adult or child is occupying the seat. However, such a weight determination can be performed only with a rather large error margin, and the results also depend greatly on temperature, so it is often impossible to differentiate clearly between an adult and a child. Since occupancy of the passenger's seat by a rear-facing child seat is especially critical, as mentioned above, the same document describes a sensor which detects unambiguously the presence of a rear-facing child seat on the passenger's seat. This sensor is based on an electromagnetic transponder principle, with transmitters and receivers provided in both the child seat and the passenger's seat. This sensor principle would require that every child seat available on the market be equipped with two resonator coils. Hence, this is a relatively expensive sensor principle.
An object of the present invention is to create a device of the type mentioned above which can be implemented with the lowest possible expense and which will identify with great reliability at least one type of occupancy of the passenger's seat for which the passenger-side air bag definitely must not be activated.